The so-called "Joint Extrusion Process" is known for the production of hollow plastic articles by inflation of a preliminary blank in a partitioned blow mold, the blank being produced in a tube forming device or tip which is fed from an extruder. In this process the tube formed in the device and expelled therefrom, after which it is crosscut to form the preliminary blanks for the subsequent blowing, is a laminate of a plurality of concentric layers. Such multiple layered preliminary blanks are used for the production of hollow articles which are often containers. The contents of such containers, composed of traditionally easily inflatable material, may diffuse into the containers' walls or otherwise become sources of problems, such as swelling. It has been suggested that other materials be used which do not react to contents, e.g. oil, fats or solvents, for production of containers. But the available selection of these materials is limited and are quite expensive in comparison with the standard materials used for production in the blow process. Accordingly, it is not generally practicable to produce such containers with thin walls because the containers must have a certain minimum strength in view of their physical and chemical characteristics to prevent ruptures, and must also have stability and rigidity which can not be attained if their walls are too thin. The joint extrusion process permits, however, the production of satisfactory containers having thinner walls. In such process, in the subsequent mold productions from blanks, a very thin so-called "blocking layer" or "barrier layer" of a diffusion-preventive and expensive material is cladded on both sides over a relatively thicker so-called "support layer" of traditional, less expensive blowable material. To guarantee a good connection between the cladding layer and the inside and outside support layers, adhesive agents or thin connection layers are often inserted between the layers, so that in practice a five-layer composite customarily results. There are many modifications of this process of little interest insofar as the scope of the instant invention is concerned, which modify the three-layer composite to a seven-layer composite by the incorporation of further intervening layers of appropriate materials.
Traditional tube forming devices used in blow technology, and also extruded tube which is used for the production of smaller containers and the like, operate continuously, i.e. heated plastic material is fed continuously through one or more extruders into the tube forming device where it is formed into tubes and from which it is continuously discharged in quantities required for units to be fed to the mold. However, this continuous tube production does not suffice in the production of larger hollow articles, for instance larger containers, for which large preliminary blanks are required. When large diameter tubes with relatively large mass which thus are of considerable weight, are expelled from the tube forming device, the weight of the material can cause breakage or deformation at the discharge end. The "dynamic air pressure or accumulating head" was therefore developed for the production of larger tubes. This device has a dynamic air pressure chamber, in which the material for production of a preliminary blank is stored until it is pressed out by a piston. The stored material is pressed out much more rapidly by the piston than would be the case with a continuous discharge of the same quantity from the reserves. The hose is formed in the accumulating head and thus the danger of its being unduly stretched or ruptured are avoided.
When a conventional piston, acting on plastic material in the reservoir chamber of the tube forming device, presses directly on the material and the reservoir chamber is filled with only one material, the piston action does not adversely affect the material or process. On the other hand, if a dynamic air pressure or reservoir chamber is filled with a plastic laminate, for example with a five-layer laminate, because then the piston may not act uniformly on the laminate, the reaction of the plastic laminate to the action of the piston during the expulsion process is more complex and the layers may buckle or the layers' composite may be otherwise adversely affected.
With a known tube forming device according to DE-OS No. 26 04 247, the individual layers produced in annular passages are expelled by an annular piston and thus are brought coaxially to the junction point forming the laminate. This, however, does not relate to a accumulating head, because a fed-in laminate is not involved. Rather each individual layer for the laminate formation is brought up to a point shortly before the nozzle. The construction is complicated, requiring, for example, five separately powered and controlled annular pistons for a five-layer laminate.